The present invention relates to an ion trap mass spectrometer, and in particular to an ion trap mass spectrometer making it possible to obtain a high sensitivity without the lowering of resolution.
As a conventional ion trap mass spectrometer, there can be mentioned a spectrometer disclosed in a literature: Analytical Chemistry 1990, Vol. 62, page 1284. In FIG. 14, a schematic structure of the ion trap mass spectrometer described in this literature is shown. In this spectrometer, an electrospray method, which is an ionizing method using an electrospray phenomenon, is used to ionize a sample solution. Ions generated by electrospray from the end of a capillary under atmospheric pressure are introduced through a differential pumping region to a vacuum region. Since the ions introduced into the differential pumping region are focused with a first electrostatic focusing lens disposed inside the differential pumping region, transmission efficiency of the ions through the differential pumping region is improved. The ions having passed through the differential pumping region are focused with a second electrostatic focusing lens, and then are introduced into an ion trap type mass analysis region 12 composed of a pair of endcap electrodes 11a and 11b in a bowl-like form and a doughnut type ring electrode 11c. The ions introduced into the mass spectrometer 12 are subjected to mass analysis by a radio frequency electric field generated by a radio frequency potential applied between the ring electrode 11c and the endcap electrodes 11a, 11b, and then are detected with an ion detector. Since in this ion trap mass spectrometer the ions are once trapped in the ion trap mass analysis region and subsequently mass analysis is carried out, the spectrometer has a characteristic that signal intensity becomes highly larger, in particular in measuring a mass spectrum.
As shown in FIG. 15, however, when the ions having passed through an aperture of a skimmer 5 are introduced into the ion trap mass analysis region 12 in the conventional spectrometer through a focusing lens 7 and a deflector, the ions are decelerated between the focusing lens 7 and the endcap electrode 11a to be defocused. Therefore, if the diameter of an ion sampling aperture 23 made in the endcap electrode 11a is small, for example, about 1 mm, transmission efficiency of the ions is lowered when the ions pass through the ion sampling aperture 23. To set the diameter of the ion sampling aperture 23 to a small value as described above is for the purpose of not disturbing the radio frequency electric field in the region surrounded by the endcap electrode 11a and the ring electrode 11c as much as possible. In other words, if the diameter of the ion sampling aperture 23 is made large, for example, about 3 mm to heighten the ion transmission efficiency in this place, disturbance of the radio frequency electric field in the ion trap mass analysis region 12 surrounded by the endcap electrodes 11a and 11b and the ring electrode 11c becomes intense, so that the peak of intensity of the ion gets broad, and resolution drops. This is the same as for an ion extracting aperture 24 made in the endcap electrode 11b. Namely, when the diameter of the ion extracting aperture 24 is large, the radio frequency electric field in the ion trap mass analysis region 12 is disturbed in the same manner as above, so that resolution drops. In FIG. 15, reference number 10 designates a gate electrode for controlling ion incidence into the ion trap mass analysis region 12; 13, an ion extraction lens for extracting mass-analyzed ions from the ion trap mass analysis region 12; 22a and 22b, insulated rings for maintaining insulation between the endcap electrodes 11a and 11b, and the ring electrode 11c; and 32, an ion detector for detecting ions extracted from the inside of the ion trap mass analysis region 12.
In the conventional spectrometer, for two purposes of filling the mass analysis region 12 with a buffer gas for trapping ions, and maintaining electric isolation between the endcap electrodes 11a and 11b and the ring electrode 11c, the insulated rings 22a and 22b made of quartz are arranged between the endcap electrodes 11a and 11b, and the ring electrode 11c, as shown in FIG. 15. Therefore, when a part of ions introduced through the ion sampling aperture 23 into the mass analysis region 12 collides with inner wall faces of the insulated rings 22a and 22b, the insulated rings 22a and 22b take a charge so that a trajectory of the ions in the mass analysis region 12 is disturbed by the charge. This results in a problem that detected ion intensity is remarkably reduced.
Furthermore, in the conventional spectrometer no measures are taken to meet a problem that as shown by an arrow in FIG. 15 a part of the ions and charged droplets having passed through the aperture of the skimmer 5 does not pass inside the mass analysis region 12 but passes through the outside thereof to reach the ion detector 32 so that noises are generated. In other words, when there are generated ions and charged droplets which make a detour through the outside of the mass analysis region 12 and then become stray in the vicinity of the ion detector 32, they are accelerated to the ion detector 32 and flow thereinto so that they are detected as random noises. Therefore, noises largely increases.
Furthermore, by the inventors' investigation, it has been found that, since a radio frequency potential is applied to the ring electrode 11c in ion trap type mass spectrometers if the stray ions and the like exist, they are accelerated by a leakage electric field as well which is generated by the application of the radio frequency potential, and the stray ions reach the detector 32. Consequently this type spectrometer has a more serious evil by the stray ions or the like than other type mass spectrometers.